Whisk the yogurt, lemon juice, minced garlic, and salt together in a small bowl and refrigerate until serving.

Toss the onions with salt in a large bowl and microwave until softened, about 5 minutes. Rinse them thoroughly under cold water, drain, and pat dry with paper towels to ensure they fry properly.

Cook the onions and vegetable oil in a Dutch oven over high heat, stirring often, until the onions are deeply golden brown, about 25 to 30 minutes. Drain the onions in a colander set over a bowl, reserving the flavored oil; transfer the onions to a paper towel–lined sheet.

Bring the lentils, water, and salt to a boil in a medium saucepan over high heat. Reduce the heat to low and simmer until the lentils are tender but still hold their shape, about 15 to 17 minutes, then drain and set aside.

Place the rice in a bowl and cover with hot tap water. Let stand for 15 minutes, then swish the grains gently to release starch. Drain and rinse with cold water several times until the water runs clear.

Heat three tablespoons of the reserved onion oil in the empty Dutch oven over medium heat. Add the garlic, coriander, cumin, cinnamon, allspice, pepper, and cayenne and cook until fragrant, about 2 minutes.

Stir in the rice and cook, stirring occasionally, until the edges of the grains begin to turn translucent, about 3 minutes. Stir in the water, sugar, and salt and bring to a boil; fold in the lentils, cover, and cook on low until the liquid is absorbed, about 12 minutes.

Remove the pot from the heat and place a clean, folded dish towel under the lid. Let it sit for 10 minutes to absorb excess steam. Fluff with a fork, stir in the cilantro and half of the fried onions, and serve topped with the remaining onions and yogurt sauce.